Bone density or bone mineral density (BMD) is the amount of bone mineral in bone tissue. The concept is of mass of mineral per volume of bone (relating to density in the physics sense), although clinically it is measured by proxy according to optical density per square centimeter of bone surface upon imaging. Bone density measurement is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk. It is measured by a procedure called densitometry, often performed in the radiology or nuclear medicine departments of hospitals or clinics. The measurement is painless and non-invasive and involves low radiation exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip. The forearm may be scanned if the hip and lumbar spine are not accessible.
Body composition is expressed by the fraction of fatty tissue composing soft (non-mineral) tissue. Body composition is used in clinical medicine as an indirect indicator and/or as a risk factor of conditions or diseases (e.g., sarcopenia, diabetes, obesity, etc.). Densitometry is also used to assess body composition. Measurements are most commonly made over the whole body or whole body less head.
Fractures of the legs and pelvis due to falls are a significant public health problem, especially in elderly women, leading to much medical cost, inability to live independently, and even risk of death. Bone density measurements are used to screen people for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.
While there are many different types of BMD tests, all are non-invasive. Most tests differ according to which bones are measured to determine the BMD result. These tests include:                Dual-energy X-ray absorptiometry (DXA or DEXA)        Dual X-ray Absorptiometry and Laser (DXL)        Quantitative computed tomography (QCT)        Quantitative ultrasound (QUS)        Single photon absorptiometry (SPA)        Dual photon absorptiometry (DPA)        Digital X-ray radiogrammetry (DXR)        Single energy X-ray absorptiometry (SERA)        
DXA is currently the most widely used and works by measuring a specific bone or bones, usually the spine, hip, and wrist. The density of these bones is then compared with an average index based on age, sex, and size. The resulting comparison is used to determine risk for fractures and the stage of osteoporosis (if any) in an individual.
As illustrated in an exemplary embodiment in FIG. 1, a DXA scanner 100 includes a table 102 for supporting a patient 101 and in which is positioned an x-ray source 104 (typically composed of an x-ray generator, an x-ray tube, an x-ray filter and an x-ray collimator) that is movable with respect to the table 102 below the patient 101. In most implementations of DXA systems/scanners 100, the detector 106 is placed within the arm 108 opposite the detector 106, such that the detector 106 and source 104 are located on opposed side of the patient 101. The detector 106 is mainly one-dimensional, but can be two dimensional or other suitable dimensional configurations, and shall be moved to capture x-ray photons emitted by the x-ray source 104 and going through the patient body 101. The arm 108 moves the detector 106 and is associated with the x-ray source 104 that is moving in synch with the detector 106 on the arm 108. The arm 108 moves both the detector 106 and x-ray source 104 in a direction corresponding to the longer dimension of the DXA table 102. In DXA scanner implementing raster scan (pencil beam or fan beam), both detector 106 and x-ray source 104 can be moved in a direction perpendicular to the longer dimension of the DXA table 102 in order to scan the table 102/body 101 along its width.
In alternative embodiments, such as shown in FIG. 1, the table 102 includes an x-ray detector 106 disposed within an arm 108 spaced above the table 102 that is movable with respect to the table 102. The table 102, along with the detector 106 and the x-ray source 104 and arm 108 are operably connected to a computer system 110 that can control the operation of the x-ray source 104 and/or arm 108, and that can receive imaging data from the detector 106 resulting from x-rays from the x-ray source 104 passing through the patient 101 and striking the detector 106.
In a DXA imaging procedure, the scanner 100 moves the arm 108 and x-ray source 104 along the portion of the body of the patient 101 to be imaged in order to obtain multiple pairs (high and low energy) of two dimensional (2D) DXA images of the specified portion of the patient. The DXA scanner 100 can move the detector 106/x-ray source 104/arm 108 along the body of the patient 101 from head to toe or along any portion of the body 101 in order to obtain the desired DXA images. Depending upon the type of beam generated by the x-ray source 104, e.g., pencil, fan or narrow fan (FIGS. 2A-2C), the x-ray source 104 and/or detector 106 and/or arm 108 can move directly along the main axis of the patient body or in a raster scan pattern in order to enable the x-ray source 104 and detector 106 to image the entire or specified portion of the body of the patient 101.
In performing the DXA imaging, the x-ray detector 106 produces dual energy (high energy (HE) and low energy (LE)) images of the specified portion of the body by detecting two different x-ray beams with different energy spectra generated by the x-ray source 104 or by detecting one x-ray beam generated from the x-ray source 104 and discriminating two different bins of energy. There are 3 main ways to implement DXA:                Two x-ray beams of different energy spectra and a detector integrating energy deposited by the transmitted x-ray photons (photons which went through the body)        One x-ray beam with a specific energy spectrum and a detector discriminating at least two beams of energy in the transmitted x-ray photons.        One x-ray beam with a specific energy spectrum and a detector composed of at least 2 layers of detector elements such as the upper layer will detect preferentially low-energy photons, and the lower layer will detect preferentially high-energy photons.        
One image is high energy and the other is low energy. The x-ray beams pass through the patient 101 being scanned and contact the detector 106 positioned on the scanner 100 opposite the x-ray source 104. The detector 106 is contacted by those x-rays passing through the patient 101 that are not absorbed by the patient tissue (bone and soft tissue), and thus measures the amount of x-rays that passes through the tissue from each beam. This will vary depending on the composition and the thickness of the tissue. Based on the difference between x-ray absorption of the tissue by the two beams, bone density and/or body composition can be measured.
In bone densitometry, the scan results are analyzed and reported as an average areal bone mineral density BMDa=BMC/A[Kg/m2], where BMC is the bone mineral content [Kg] and A is the projected area [m2] of the volume containing the mixture of which bone mineral is part. The results are generally scored by two measures, the T-score and the Z-score. The Z-score denotes the difference between a measured value BMDa in an individual subject and the age-matched mean reference value normalized by the age-matched standard deviation of the population variance. The T-score is defined similarly but instead of age-matched values data from the young reference population are used. Concrete diagnostic criteria solely based on BMD have been provided by the operational definition of osteoporosis based on the T-score, developed by a working group of the WHO. The WHO (1994) definition uses areal BMDa as measured by DXA to categorize a subject into one of four groups: Normal (BMDa T-score≥−1.0); Low bone mass or osteopenia (−1.0>BMDa T-score>−2.5); Osteoporosis (−2.5≥BMDa T-score); Established osteoporosis (−2.5≥BMDa T-score and at least one osteoporotic fracture.
Use of DXA for BMD has several limitations. For example, as the calculation of bone density is only an approximation of bone strength based on the calculated mineral density of the bone, it is desirable to have an indication of the stress and/or strain on the bones at any location on the bone(s), in order to provide a more direct indicator of bone strength at those locations. One approach to provide this bone strength analysis is the combination of the DXA image with finite element analysis to provide a more detailed representation of the bone density of the patient.
To assist the DXA image/process in determining BMD, in DXA generally 2 images are produced, namely, a high-energy (HE) image and a low-energy (LE) image. The HE & LE images can be combined by software to generate one image of bone-equivalent thickness and one image of soft-tissue equivalent thickness. Other pairs of tissue-equivalent images can be derived from the HE & LE images providing a material calibration derived from data acquired on physical or simulated phantoms made of the adequate materials. In addition, it is also possible to use only one of the 2 images (HE or LE, Bone or soft-tissue). In the illustrated exemplary embodiment of FIG. 3, once the at least one 2D DXA image is obtained, using a suitable computer system 110/50 the at least one DXA image 10 can be compared with a reference database 12 of stored 2D DXA images 14 operably connected to or contained on the computer system 50 in order to locate a at least one stored 2D DXA image 14 that best approximates or is most similar to the at least one obtained 2D DXA image 10. One example of a system 50 and database 12 of this type is found in the 3D-DXA software package available from Galgo Medical SL, of Barcelona, Spain, in which the system 50 reconstructs bony structures in 3D from 2D DXA images to assess the cortical bone and trabecular macrostructure of the bone in the DXA image 10 according to a process disclosed in Humbert L, Martelli Y, Fonolla R, et al 2017 3D-DXA; Assessing the Femoral Shape, the Trabecular Macrostructure and the Cortex in 3D from DXA images. IEEE Trans Med Imaging 36: 27-39., which is expressly incorporated by reference herein in its entirety for all purposes. In this process, the reference database 12 includes the stored DXA images 14 and stored CT scan sets of images 18 that have previously been obtained from other patients and corresponding to DXA images 14. The stored DXA images 14 and corresponding stored CT sets of images 18 are utilized by the system 50 to construct a set of 3D finite element analysis (FEA) model 20 which is also stored in the database 12, each FEA model in association with each corresponding DXA image 14 and CT scan set of images 18 from which the model 20 was derived. When the system 50 is presented with the at least one obtained DXA image 10, the system 50 locates the 2D DXA image 14 best approximating the at least one obtained 2D DXA image 10. The system 50 then locates the FEA model 20 associated with the particular stored DXA image 14 selected as being closest to the at least one obtained DXA image 10. The system 50 then modifies the FEA model 20 based on the parameters from the at least one obtained 2D DXA image 10 in order to arrive at a modified FEA model 20′ that illustrates different color-maps on the model 20′ of different parameters relating to the bone imaged with the DXA scanner providing the at least one DXA image 10, including but not limited to, cortical thickness, cortical volumetric density and trabecular volumetric density, among others.
Alternatively, as shown in FIG. 4, the system 50 can be operated to compute the stored 2D DXA images 16 associated with a particular CT scan set of images 18 directly from the CT scan set of images 18. These computed 2D DXA images 16 are then compared with the at least one obtained 2D DXA image 10 in the manner described previously in order to arrive at the modified FEA model 20′.
However, while the process of obtaining the modified FEA model 20′ provides additional information about the bone structure in the at least one obtained 2D DXA image 10, the process for constructing the modified FEA model 20′ still has significant shortcomings in that the at least one obtained 2D DXA image 10 contains a limited amount of information that can be utilized to modify the FEA, model 20 and provide an accurate representation of the structure of the bone(s) of the patient.
Accordingly, it is desirable to provide an imaging system and method for determining bone density and other associated parameters with the capability to provide an operator with enhanced volumetric imaging capabilities, thereby improving the scan results and providing a better measurement of bone density and strength.